Slag which is used in the manufacture of mineral fibre for insulating purposes may contain a number of mineral substances, and the main constituents are usually in the following descending order of concentration, namely, calcium oxide, silicon dioxide, aluminium oxide, magnesium oxide, and iron oxide. For satisfactory melting of this slag it is necessary to run the furnace of temperatures which may reach as high as 1,600.degree. C., and considerable difficulty is encountered in the discharging of such a high temperature melt.
If the melt within the furnace adjacent a discharge opening is allowed to cool it will at least partly solidify and the solidified slag needs to be broken before the melt will again flow outwardly through the opening. This is very inconvenient, and in some instances can be dangerous. This is one of the main reasons why the furnaces which are used for the melting of slag are usually combustion furnaces, containing slag and coke, and being surrounded by a water jacket. Air is introduced through tuyeres in or surrounding the lower wall of the furnace, and as the coke in the furnace burns, the high temperature melts the slag which trickles down through the coke and runs out as a continuous hot stream from an opening in or near the base of the furnace. However, the nature of the furnace is such that the flow rate is widely variable, dependant upon the conditions of combustion, and if the discharging slag is run onto a spinning disc for example for the manufacture of fibrous material for insulation purposes, a wide variation of quality results.
In the alternative, if an electric furnace is used, there are a number of advantages including reduction of pollution and a much cleaner operation. However, an electric furnace, if used for continuous production, will contain a body of molten slag in the base of the furnace, and it is desirable that the flow rate of discharge of the melt should be carefully regulated without the slag chilling to such a degree that it partly solidifies.
In the U.S. Pat. Nos. 2,790,019 (STALEGO), 686,836 (RUTHENBURG) and 1,202,837 (HECHENBLEIKNER) there are disclosed various water cooled jackets for the discharge openings of furnaces, and STALEGO also discloses the use of a graphite block over which the melt of an electric furnace can discharge in an overflow manner. The STALEGO furnace is useful for batch production, but quite unsuited for continuous production of a melt such as is required for the production of mineral fibre.
As far as is known, prior art does not include any furnace valve which is suitable for controlling a flow rate of fused slag (or other oxide compounds of metals/non-metals), being discharged from the body of melt contained in an electric furnace, yet accurate control of flow rate is of importance in the maintaining of constant quality of product.
The main object of this invention is to provide a furnace valve which is useable to accurately control such flow rate under continuous production conditions.